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selinux: Use a better hash function for avtab

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This function, based on murmurhash3, has much better distribution than
the original. Using the current default of 2048 buckets, there are many
fewer collisions:

Before:
101421 entries and 2048/2048 buckets used, longest chain length 374
After:
101421 entries and 2048/2048 buckets used, longest chain length 81

The difference becomes much more significant when buckets are increased.
A naive attempt to expand the current function to larger outputs doesn't
yield any significant improvement; so this function is a prerequisite
for increasing the bucket size.

sds:  Adapted from the original patches for libsepol to the kernel.

Signed-off-by: John Brooks <john.brooks@jolla.com>
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>
Signed-off-by: Paul Moore <pmoore@redhat.com>
This commit is contained in:
John Brooks 2015-03-24 16:54:17 -04:00 committed by Paul Moore
parent ba39db6e05
commit 33ebc1932a
2 changed files with 38 additions and 5 deletions
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41
security/selinux/ss/avtab.c
View file

@ -25,10 +25,43 @@
static struct kmem_cache *avtab_node_cachep;
static inline int avtab_hash(struct avtab_key *keyp, u16 mask)
/* Based on MurmurHash3, written by Austin Appleby and placed in the
* public domain.
*/
static inline int avtab_hash(struct avtab_key *keyp, u32 mask)
{
return ((keyp->target_class + (keyp->target_type << 2) +
(keyp->source_type << 9)) & mask);
static const u32 c1 = 0xcc9e2d51;
static const u32 c2 = 0x1b873593;
static const u32 r1 = 15;
static const u32 r2 = 13;
static const u32 m = 5;
static const u32 n = 0xe6546b64;
u32 hash = 0;
#define mix(input) { \
u32 v = input; \
v *= c1; \
v = (v << r1) | (v >> (32 - r1)); \
v *= c2; \
hash ^= v; \
hash = (hash << r2) | (hash >> (32 - r2)); \
hash = hash * m + n; \
}
mix(keyp->target_class);
mix(keyp->target_type);
mix(keyp->source_type);
#undef mix
hash ^= hash >> 16;
hash *= 0x85ebca6b;
hash ^= hash >> 13;
hash *= 0xc2b2ae35;
hash ^= hash >> 16;
return hash & mask;
}
static struct avtab_node*
@ -256,7 +289,7 @@ int avtab_init(struct avtab *h)
int avtab_alloc(struct avtab *h, u32 nrules)
{
u16 mask = 0;
u32 mask = 0;
u32 shift = 0;
u32 work = nrules;
u32 nslot = 0;

2
security/selinux/ss/avtab.h
View file

@ -56,7 +56,7 @@ struct avtab {
struct flex_array *htable;
u32 nel; /* number of elements */
u32 nslot; /* number of hash slots */
u16 mask; /* mask to compute hash func */
u32 mask; /* mask to compute hash func */
};